Implement attribute waitingforkey on audio/video elements.
IDL attribute was added in bug 1304247 but we are missing corresponding
content attribute: https://w3c.github.io/encrypted-media/#attributes-3
Differential Revision: https://phabricator.services.mozilla.com/D226298
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1925952
gecko-commit: 4c149b703e82d202868d190a514090eb31f14a4b
gecko-reviewers: smaug